The invention concerns a method and device for statically or dynamically determining set values for braking forces or braking torques.
The invention is designed for application in vehicles that are driven completely or at least partially by electric drives. This type of motor vehicle is shown schematically in FIG. 1. Via a transmission, an electric drive, for example an electric motor 101, can act on a vehicle axle 110b, preferably the front axle, and drive its wheels 109c, 109d. The motor receives its energy from a battery 102; however, a switch block 112 is arranged in between them, and it switches the energy supply according to a control system. The switch block 112 can exhibit comparatively simple switches, for example relays, or more complex switches, for example thyristors.
Since, in principle, electric motors can function as generators, electrodynamic regenerative brakes already have been proposed for electrically driven vehicles. In that case, the motor is no longer used as the drive, but as a generator that is driven by the momentum of the vehicle by way of the wheels and the drive train and, consequently, charges the battery 102.
For various reasons, such a brake system alone may not provide sufficient performance under all driving conditions. For example, at low speeds, the electric motor may not generate enough power to build up the required braking force as a generator. In addition, especially in connection with asynchronous transmissions, no manual transmission is used and, therefore, there is not sufficient braking force at high speeds, i.e. high r.p.m. The relationship P=Mxc2x7xcfx89 describes the relationship between motor power P, motor torque M and motor speed xcfx89. If power P is considered to be more or less constant, then the available drive torque and, consequently, also the available braking torque are low at high speed. Thus, an electrodynamic regenerative brake often is not powerful enough at high speeds to satisfy the actual or legal requirements. The same applies with respect to medium speed ranges when high deceleration is necessary. Despite the low speed, the electrodynamic regenerative brake sometimes may not provide adequately high braking torque. Another reason for the insufficient performance of a regenerative brake system is the limited chargeability of the batteryxe2x80x94for example in that operating state when the battery has just been fully charged and no additional consumers of electric power are provided, and no braking torque is generated by the electric motor acting as a generator. The battery can be destroyed by overcharging.
Hence, it has been proposed to supplement the brake system of a vehicle driven additionally by electric means by a more or less conventional brake system. Thus, for example, EP-A-0 361 708 studies a combination of an electrodynamic regenerative brake and a conventional friction brake. In this case, the friction brake is used for providing the braking torque that may be missing; however, the system is designed to maximize the braking torque generated by the electrodynamic regenerative brake in order to keep the amount of recycled electric energy as high as possible. This system, however, does not take into consideration adequately the required distribution of the braking force on the front axle and rear axle of a vehicle. In particular when the driver applies the brakes strongly, the braking forces have to be distributed on the vehicle axles in such a way that optimal braking results are achieved.
Furthermore, apart from the statically obtained set value, an electrodynamic regenerative brake on the one hand and a conventional brake, e.g. an hydraulic brake, on the other hand have different dynamic response characteristics. If the cooperation of the two brakes is to be optimized, the different dynamics of the two brakes have to be adapted to each other.
The object of the present invention is to disclose a method and a device for determining set values for braking forces that lead to well adapted braking procedures.
Another object of the invention is to disclose a method and a device for determining set values for braking forces or braking torque that take into consideration the dynamic characteristics of the brake systems.
Prior to describing the invention, the terms used in this application shall be explained. When only xe2x80x9cset valuesxe2x80x9d are mentioned, this refers to set values for braking forces or braking torque. In general, therefore, high set values indicate a strong application of brakes and low set values point to a weak application of brakes. Moreover, only braking torque is referred to below; braking force is equivalent to this.
The invention relates to a vehicle comprising both an electrodynamic regenerative brake and a friction brake. Whereas the friction brake acts on all wheels of a vehicle, the electrodynamic regenerative brake acts on at least one axle of the vehicle. If, for example, an electric vehicle has front-wheel drive, the electrodynamic regenerative brake can act only on the front axle. The electrodynamic regenerative brake works according to the mechanism described above, i.e. the driving electric motor is used as a generator for charging the battery. The torque required to drive it is then supplied to it through the push of the vehicle that occurs during braking/deceleration via the drive train (wheels, axle, transmission). Consequently, the electrodynamic regenerative brake can only act on that axle, preferably the front axle 110b, that is driven by the electric drive, e.g. the electric motor 101. This may be acceptable when the brakes are applied weakly, since the braking torque generated by one axle alone will be adequate. As a matter of fact, it is desirable because a high quantity of electric energy can then be recycled, particularly in view of the fact that significantly more than 70% of all braking actions are comparably weak brake applications. Thus, as far as weak brake applications are concerned, a concept can be applied which ensures that the electrodynamic regenerative brake maximizes the braking procedure and that accordingly maximum set values according to the braking requirement are supplied to this brake.
In connection with strong brake applications, however, different conditions have to be taken into account. The braking torque must be applied to both axles since then the friction value can be used on all wheels of the vehicle for deceleration. Thus, optimal utilization of the friction value on both axles can be achieved.
For another it may be necessary to do without the criterion of maximizing the set values for the electrodynamic regenerative brake, for example when this would cause strong braking of the wheels under review. On the other hand, it may be possible that the technically possible maximum braking torque is not sufficient for the electrodynamic regenerative brake to reach the set value specified for the respective axle. Then the missing quantity has to be replaced by the friction brake or its set values.
Hence, the strategy of maximizing the brake power of the electrodynamic regenerative brake is combined with a strategy of optimizing the braking characteristics by distributing the braking forces on the different axles of the vehicle. Which of these strategies should be adopted is determined by the deceleration requirement, for example on the basis of a desired vehicle deceleration determined from the deceleration requirement. In a first range of the desired vehicle deceleration with comparably low values, the strategy of maximizing the brake power of the electrodynamic regenerative brake can be followed. This frequently means that only the electrodynamic regenerative brake is used. However, in a second range of the desired vehicle deceleration with higher deceleration values, the strategy of distributing the braking force on the axles of the vehicle is followed.
The limit between the two ranges can be variable, with such change being based on either internal or external operating conditions.
The deceleration requirement that influences which braking strategy is chosen can be the driver""s braking requirement as it is indicated through the brake pedal, for example. In the same way, however, deceleration requirements generated by the control system can be processed, for example from a follow-up control or similar system.
The deceleration requirement can comprise a desired vehicle deceleration (dimension m/s2, negative acceleration). For one, if the deceleration requirement is derived from the brake pedal, this value in its first approximation would correspond to the position of the brake pedal. For another, the dynamicsxe2x80x94the time-related changexe2x80x94of the deceleration requirement (dimension m/s3) can be studied, too. If the deceleration requirement is derived from the brake pedal, this would correspond to the actuation speed of the brake pedal in the first approximation. The dynamics of the deceleration requirement indicate whether a soft, xe2x80x9cnormalxe2x80x9d braking operation or a sudden, xe2x80x9cunusualxe2x80x9d panic braking operation is to be executed. Thus, for example according to the dynamics of the deceleration requirement, the above-mentioned delimitation of ranges can be shifted in favor of the braking strategy of distributing the braking torque on the axles of the vehicle, even if the desired vehicle deceleration still has comparably low values.
The friction brake preferably is an hydraulically actuated brake that can act on all wheels of the vehicle individually. Particularly advantageous is a xe2x80x9cbrake-by-wirexe2x80x9d system, wherein the brake pressurexe2x80x94and accordingly the braking torquexe2x80x94is built up hydraulically at the individual wheels but individually according to electric signals from a control system. With such a system a different braking torque for the friction brake can be built up in a simple manner wheel by wheel or axle by axle. The xe2x80x9cbrake-by-wirexe2x80x9d system can also have an electromechanical construction.
Furthermore, it has become evident that the dynamics of the friction brake on the one hand and those of the electrodynamic regenerative brake on the other hand are different. In particular the electrodynamic regenerative brake frequently exhibits slower response characteristics than the friction brake. This can be attributed on the one hand to relative long transmission distances in the drive train, on the other handxe2x80x94and in particularxe2x80x94to the control system. Since practical embodiments of the combined systems described above (combinations of friction brake and electrodynamic regenerative brake) called for a friction brake control as well as a control system for the electrodynamic regenerative brake, dead times can arise in the signal processing, and this is why the electrodynamic regenerative brake responds slower. In the same way, however, switching the motor between driving and braking operation can lead to dead times, since the time constants of motor inductance in particular do not permit quick switching. Thus, the electrodynamic regenerative brake often is slower to reach its specified set value than the friction brake.
For this reason it is proposed that at least one set value for the friction brake (generated according to static criteria) be modified according to the dynamic characteristics of the electrodynamic regenerative brake. In particular, that set value of the friction brake that is intended for the vehicle axle on which the electrodynamic regenerative brake acts can be modified. The modification or correction, for example, can be executed through a control approach or by applying a simulation.